Earth drilling rigs, of the kind used to drill water wells, and for mineral exploration, etc., often incorporate a rotary screw air compressor to provide air for the purpose of flushing cuttings from the borehole. In some cases the compressor is also used to provide compressed air for the operation of a down-the-hole hammer for percussive drilling of hard rock.
Drilling rig air compressors are typically regulated by pneumatic controls adapted from general purpose air compressors of the kind used in construction. An air-actuated throttle valve is provided at the compressor's air inlet to control the flow of air through the intake of the compressor. When the pressure in the compressor's air receiver reaches a preset upper limit, the throttle valve is closed, and the compressor is “unloaded,” that is, it effectively stops compressing. When the pressure in the receiver falls below a preset lower limit, the valve opens, and the compressor resumes its operation. Thus, the compressor continually switches between a loaded condition and an unloaded condition, operating in an “on-off” mode. In the closed, or unloaded, position, an orifice in the throttle valve allows a small amount of air to enter the compressor. The throttle valve is “substantially” closed, and the volume of air being compressed is only that necessary to avoid cavitation.
The volume of air delivered by the conventional compressor, that is, the volume flow rate, usually measured in cubic feet per minute (cfm), is fixed when the compressor is loaded, that is, when the compressor intake throttle valve is open. There are no intermediate valve positions. Therefore, when the required air volume is less than the full compressor volume capability, the compressor unloads more frequently.
To be powerful enough for effective drilling, yet compact enough to be moved over public highways from job to job, a drilling rig typically employs a single internal combustion engine to power both the compressor and one or more hydraulic pumps which supply hydraulic fluid for the operation of various hydraulic motors and hydraulic actuating cylinders. The hydraulic motors and cylinders are used for various purposes, including rotation of the drill bit, feeding of the bit into the borehole, lifting the drill pipe, operation of devices used to handle the drilling tools, and performance of other drilling rig functions.
In the course of drilling, the power required from the engine by the hydraulic pumps varies according to the size of the hole being drilled, the formations encountered, the amount of water in the hole, etc. Power for the air compressor also varies according to the amount of air required to flush the hole of cuttings and the amount of air required to operate a down-the-hole hammer, when one is used. The engine, air compressor, hydraulic pumps, and other elements of the drilling rig, interact to determine the quality of the hole and the efficiency with which it is drilled. A large volume of compressed air is required for drilling large diameter boreholes, and increases the drilling penetration rate in the case of smaller diameter boreholes. Therefore, in general, drilling contractors desire an air compressor that produces a large volume of compressed air. However, some geological formations cannot tolerate a large volume of air because it can cause borehole erosion. Borehole erosion is detrimental to borehole quality, and can cause deterioration of the casing-to-earth seal, undermining of the drilling rig outriggers, and total borehole collapse or cave-in. On a conventional drilling rig, with a general purpose air compressor control system, the compressor output cannot be matched to the borehole air flow.
Under certain combinations of conditions, the power requirement may exceed the power available, causing the engine to become overloaded and stall. If the engine stalls, the borehole flushing medium is lost, and the hydraulic power to turn and feed the bit is also lost. This can cause a host of problems in the borehole, such as borehole cave-in, backfill, a stuck bit, etc.
In addition, during drilling, because the power drawn by the compressor increases and decreases as the compressor is continually loaded and unloaded, the engine speed can vary considerably, and the hydraulic power available for drilling functions varies, causing erratic operation of the various hydraulically powered devices. Continual loading and unloading of the compressor also raises the noise level at the operator's station. Moreover, the pneumatic components of the compressor control system are subject to malfunction as a result of frozen condensate and other contamination.
In short, general purpose air compressor controls cannot adjust a compressor which is part of a drilling rig system so as to achieve optimum drilling performance.